Luffy puts finding his crew on hold in favor of rescuing his brother Ace from the underwater prison Impel Down. With Boa Hancock's help, he manages to infiltrate the facility. However, fulfilling his objective and escaping the hellish prison might be easier said than done.

Having failed to reach Ace before he was transferred, Luffy and his companions escape from Impel Down and travel to Marineford, the base of Marine Headquarters. With the odds stacked high against him, the imminent threat of Whitebeard, and the ulterior motives of pirates that wish to overthrow the Emperor, Luffy struggles to hold his own in this titanic clash of powers.